The operation of rotary kilns for the heat processing of various particulate materials, such as kaolin and bauxite to produce mullite, is extremely energy intensive. In order to have a satisfactorily operating kiln, it is necessary that the kiln first be brought up to temperature and then be operated continuously for long periods of time at the prescribed temperature. It is the general practice to fire the kilns by means of a burner disposed at the material discharge end of the kiln to project fuel and air generally coaxial of the kiln so as to provide an intense flame in the kiln to generate the required high temperature. The primary fuel generally employed is natural gas with oil also being utilized. Additionally, minor amounts of pulverized coal have been employed. However, it is necessary that the burner and firing system be such that the product passing down through the kiln is in no way contaminated with coal firing providing a problem in this respect. Because of the large amount of fuel required in the continuous operation of a kiln, the cost with respect to the primary fuel is a very important consideration. The utilization of gas and oil results in a very high kiln operating cost.